Microporous Zones Research Articles

Dynamics of adsorption over a wide range of breakthrough concentrations. Communication 7. Dynamics of adsorption on a biporous adsorbent

1. An analytical solution has been found for the model of mixed-diffusion adsorption dynamics on a biporous adsorbent in the case of a rectangular isotherm with account of three kinds of resistance to mass transfer: external, in transport pores, and in microporous zones. 2. In the region of small relative breakthrough concentrations the final curves in the coordinates In y-t are linear. The loss of time of protective action in Shilov's equation is a linear function of the relaxation times which are characteristic for three different kinds of mass transfer.

Microporous carbon adsorbent systems Communication 3. Experimental estimation of micropore system parameters

1. A method has been developed for the experimental determination of the characteristic dimensions of the mesopore and micropore zones of active carbons based on precipitation of carbon within the adsorbent granules and subsequent grinding and measurement of its adsorption properties and the external surface after grinding. 2. In the AC-30 active carbon studied, the size of the unit micropore zones was 220 nm and that of the unit mesopore zones was 540 nm.

Micropore systems in carbon adsorbents

1. A simple model in which adhering and interpenetrating carbon crystallites form a series of micropore zones is proposed for use in discussing pore systems in carbon adsorbents. 2. Finite-dimensional micropores are formed by burn-off of crystallite carbon atoms in the course of gas-phase oxidative activation (intracrystalline micropores) and by the development of interstices between adhering crystallites (intercrystalline micropores). 3. The model micropore zone is assumed to have the form of a small cube. The fissure interstices between faces of these cubes, or micropore zones, represent the micropore volume, and the total wall surface area of these pores the mesopore surface area. Calculation of the values of the parameters of such micropore structures from minimal experimental data is illustrated for various actual adsorbents. 4. The attempt is made to model the structures of micropore zones and estimate the values of their parameters.

Microporous structures of carbonaceous adsorbents

The simplest model of the porous structure of carbonaceous adsorbents is proposed, in which the microporous zones are formed by the totality of contacting and merged carbon crystallites. The micropores limited in extent resulted from the burnout on activation of several atomic layers of carbon in the crystallites by gaseous oxidizers (intercrystallite micropores) and from some development of the spaces between contacting crystallites (intercrystallite micropores). The micropore zones are shaped into regular cubes. The slit-like spaces between part of the micropore zones represent the micropore volume, while the total surface area of the walls of such pores is the mesopore surface. The parameters of the model for typical samples of microporous carbonaceous adsorbents are calculated and discussed. The formation of intercrystallite slit-like micropores as a result of the burnout of carbon atomic layers in crystallites on avtivation is considered. It is shown that only a wide micropore distribution leading to the formation of the supermicropores practically results in deviation from the applicability of the one-term adsorption equation of the theory of volume filling of micropores.

Kinetic curves for a biporous sorbent with linear sorption isotherms with allowance for sorption in transport pores

1. We have obtained exact analytical expressions for kinetic curves and for the distribution of local sorbate concentration in biporous sorbent granules in the form of a membrane, cylinder, or hollow cylinder, in the case of linear sorption isotherms and an exponential kinetic function for microporous zones. 2. The theoretical expressions that we have found can be used to interpret kinetic sorption tests of granulated zeolites, activated carbons, ion exchangers, and two-phase polymers (block copolymers).

Limiting-stage method in the dynamics of sorption processes

1. An approximate solution to the problem of the dynamics of sorption on a bidisperse sorbent has been obtained for interdiffusion kinetics and a linear isotherm. 2. A method of calculating the parameters of the system for the grains of the microporous zones has been developed using the solutions for homogeneous sorbents.

Theory of diffusion and sorption of various substances through a biporous sorbent membrane for the case of a constant concentration difference at its boundaries and a linear sorption isotherm. I. Case of an exponential kinetic function for the microporous zones

1. An analytical solution has been obtained for the passage of the sorbed substance through a membrane of a biporous sorbent for the case of constant concentration differences of the sorptive on its boundaries and a linear sorption isotherm. 2. The distribution of the sorptive concentration in the membrane in different moments of time has been calculated as an example, for the case of equal characteristics of diffusion relaxation times in the transporting porous system and in the microporous zones.

A polarized light and microradiographic study of enamel in human primary teeth from a high fluoride area

The enamel structure of primary teeth from a high fluoride area (4–6 parts/10 6) was compared with that of primary teeth from a low fluoride area (< 0.2 parts/10 6). Macroscopically, varying degrees of enamel mottling, including pitting, were recorded in teeth from the high fluoride area. In polarized light, the enamel formed in a low fluoride area exhibited a microporous zone confined to the prenatal enamel. The slightly-affected fluorosed teeth showed a widening of this inner zone, often combined with a tendency to increased subsurface porosity particularly in the cervical portion of enamel. The subsurface zone became more pronounced, corresponding to increasing degrees of fluorotic mottling. In the cervical enamel, porosity involved the full width of the enamel whereas elsewhere it formed a distinct zone in the outer third or half of the enamel. Microradiographically. this zone appeared hypomineralized beneath a well-mineralized surface layer. Quantitation of the birefringence along traverses from the surface to the enamel-dentine junction disclosed that the optical behaviour of these enamel defects was similar to that described in permanent teeth.